1. Field of the Invention
The present invention relates generally to a control system for an internal combustion engine (hereinafter also referred to simply as the engine) and more particularly to an engine control system including an exhaust gas recirculation flow control apparatus which is capable of controlling electrically and/or electronically a recirculation flow of an exhaust gas of an engine.
2. Description of the Related Art
For having a better understanding of the present invention, description will first be made of a hitherto known engine control apparatus of which improvement is contemplated by the present invention.
FIG. 5 shows schematically an arrangement of a hitherto known engine control system in which a cylinder pressure sensor is employed for detecting a pressure within a combustion chamber (hereinafter referred to as the cylinder pressure) of an engine, as is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 253543/1989 (JP-A-1-253543). Referring to the figure, an engine body 1 includes a plurality of cylinders each of which has a cylinder head 2 equipped with a cylinder pressure sensor 3 and a cylinder temperature sensor 4 in such disposition that the respective sensor elements are exposed to the interior of the associated cylinder. A fuel injector 6 is installed in an intake port 5 communicated to each cylinder of the engine 1, the intake port 5 being additionally connected to a throttle chamber 8 by way of an intake manifold 7. The upstream side of the throttle chamber 8 is communicated to an air cleaner 10 via an air suction pipe 9. The engine 1 further includes a distributer 11 operatively coupled to a cam shaft (not shown) of the engine, which distributer in turn is provided with a timing sensor 12 for detecting predetermined crank angles of the individual cylinders. On the other hand, an air-fuel ratio sensor 15 is disposed at a conflux point of an exhaust gas manifold 14 which is communicated with discharge ports 13 of the engine 1. A catalytic converter 16 is disposed at an exit port of the exhaust gas manifold 14. A throttle valve 17 is mounted within the throttle chamber 8. An engine control unit (also referred to as ECU in abbreviation) 18 which is in charge of an overall engine control is realized by a microcomputer which in turn is constituted by a CPU (Control Processing Unit), a RAM (Random Access Memory), a ROM (Read-Only Memory), an input/output interface and others. The engine control unit ECU 18 has input terminals to which outputs of the various sensors 3, 4, 12 and 15 are electrically connected and output terminals to which the fuel injector 6 and spark plugs 21 are electrically connected via associated driver circuits 19 and 20. The spark plugs 21 are fixedly mounted on the cylinder heads 2 of the individual cylinders, respectively.
An amount G.sub.a of air charged in each cylinder can be arithmetically determined by the ECU 18, for example, in accordance with the following expression (1): EQU G.sub.a =(P.times.V)/(R.times.T) (1)
where P represents a cylinder pressure measured or determined by the ECU 18 at a predetermined crank angle (e.g. BTDC 90.degree. CA indicating a crank angle of 90.degree. before the top dead center) during a compression stroke of each cylinder, the crank angle in turn being determined on the basis of the signal supplied from the timing sensor 12. Further, V represents a volume of the combustion chamber at the predetermined crank angle mentioned above, R represents a gas constant effective during the compression stroke, and T represents the intra-cylinder gas temperature which can be determined on the basis of the output signal of the cylinder temperature sensor 4.
On the other hand, it is taught in JP-A-59-221433 that the air charge amount G.sub.a bears a linear relation to a cylinder pressure difference .DELTA.P, as shown in FIG. 7. In this conjunction, the cylinder pressure difference .DELTA.P represents a difference in the cylinder pressure between the bottom dead center (BDC) and a position corresponding to a crank angle of 40.degree. CA before the top dead center (i.e. BTDC 40.degree. CA) in the compression stroke, as can be seen in FIG. 6. An intake air amount can arithmetically be determined on the basis of this difference in the cylinder pressure between the two predetermined crank angles in the compression stroke in accordance with the linear relationship mentioned above.
Further, there is disclosed in JP-A-60-47869 a method of determining a fuel injection timing by consulting a two-dimensional fuel injection timing map table stored previously in a ROM of the engine control unit or ECU and containing data of the cylinder pressure difference .DELTA.P with engine rotation speed N (rpm) being used as a parameter, as is illustrated in FIG. 8.
The ECU 18 executes the process for calculating or determining the charged air amount G.sub.a of the engine in the manner described above. On the basis of the charged air amount determined in this way, a fuel injection pulse width T.sub.i is computed in accordance with the following expression (2): EQU i T.sub.i =K.times.G.sub.a .times.K.sub.FB (2)
where K represents an air-fuel ratio constant and F.sub.FB represents an air-fuel feedback correction quantity. On the basis of the fuel injection pulse width as calculated, the ECU 18 supplies a signal to the driver circuit 19 for driving the fuel injector 6, to thereby effectuate the air-fuel ratio control.
Additionally, there is disclosed in JP-A-59-103965 a method of determining the ignition timing with the aid of a two-dimensional ignition timing map table containing ignition time points determined previously for every engine operation state on the basis of the cylinder pressure value and the engine rotation speed (rpm) by measuring the cylinder pressure in terms of the absolute value at a crank angle of 45.degree. after the bottom dead center (i.e. at ABDC 45.degree. CA). Determination of such ignition timing can of course be executed by the ECU 18, which then supplies a corresponding drive signal to the spark plug driver circuit 20 for causing the spark plug 21 to generate a spark.
The hitherto known engine control systems of the structures described above suffer from various problems mentioned below. First, it must be pointed out that in any one of the known control systems, no measures are adopted for coping with reduction of nitrogen oxides contained in the exhaust gas. Secondly, the conventional exhaust gas recirculation flow control apparatus (not shown in FIG. 5) is so implemented as to control mechanically the exhaust gas recirculation by measuring the actual exhaust gas recirculation ratio. Besides, it is impossible with the known systems to confirm definitely whether or not an exhaust gas recirculation ratio which is appropriate for the current engine operation or running state has been attained. Additionally, it is also impossible to detect occurrence of failure or abnormality in the recirculation ratio control due to degradation in operation of the exhaust gas recirculation flow control apparatus (e.g. decrease of the gas flow through the apparatus due to deposition of smudges in the piping).